Smith's explanation is here. As near as I can tell, the problem is that he made some pretty dramatic claims about Hezbollah activity that were couched as eyewitness reports, when in reality they were based on quick automobile drive-bys combined with tips from anonymous sources:

In retrospect, however, this is a case where I should have caveated the reporting by saying that I only witnessed a fraction of what happened (from a moving car), with broader details of what I saw ultimately told to me by what I considered then  and still consider to be  reliable sources within the Cedar Revolution movement, as well as insiders within the Lebanese national security apparatus. As we were driving through that part of town, I saw men I identified as Hezbollah deployed at road intersections with radios. I was later told that these were Hezbollah militants deploying to Christian areas of Beirut, and there were four or five thousand of them.

Since then, I have not been able to independently verify that "thousands" of armed Hezbollah fighters deployed to the Christian areas of Beirut in late September, but my sources continue to insist that it happened.

Well, we all make mistakes. Live and learn, eh? Whether the New Republic will be so charitable remains to be seen.